BSCS - DCCN - F20 - Week 6 - Sec A
BSCS - DCCN - F20 - Week 6 - Sec A
BSCS - DCCN - F20 - Week 6 - Sec A
Computer Networks
Week # 06
Powerpoint Templates
This Week Course Plan
Transmission Modes
Circuit switched and packet switched Networks
Ethernet LAN Standard
Overview of Wireless LAN
ACKNOWLEDGMENT
These lecture slides contain material from slides prepared
by Behrouz Forouzan for his book Data Communication
and Networking (4th/5th edition).
TRANSMISSION MODES
The transmission of binary data across a link can be
accomplished in either parallel or serial mode
In parallel mode, multiple bits are sent with each clock
tick.
In serial mode, 1 bit is sent with each clock tick.
Parallel transmission
Use n wires to send n bits at a time; each bit has its
own wire, so all n bits of one group can be transmitted
with each clock tick between devices.
Advantage: speed (by a factor of n over serial)
Disadvantage: Cost
Serial transmission
One bit follows another which requires only one channel for communication
between devices
Advantage: reduces cost ( only one channel instead of n)
Overhead: Because communication within devices is parallel, we need
conversion devices at the interface between sender and the line (parallel to serial)
and between the line and the receiver (serial to parallel)
Serial transmission can be asynchronous, synchronous and Isochronous
Asynchronous transmission
In asynchronous transmission, signal timing is not important
for synchronization
we send 1 start bit (0) to alert the receiver to the arrival of a
new group, and
1 or more additional stop bits (1s) at the end of each byte to
let the receiver to know that byte is finished
There may be a gap between each byte
Asynchronous here means “asynchronous at the byte level,
but the bits are still synchronized;
their durations are the same
Asynchronous transmission
Slower Transmission due to additional bits
Cheap and effective
Used for low speed communication (between keyboard and
computer)
Synchronous transmission
In synchronous transmission,
we send bits one after another without start or stop bits or
gaps
It is the responsibility of the receiver to group the bits
The bits are usually sent as bytes and many bytes are grouped
in a frame
A frame is identified with a start and an end byte and there are
uneven gaps between frames
Isochronous
In isochronous transmission
we cannot have uneven gaps between frames
Transmission of bits is fixed with equal gaps
Used in real-time audio and video where uneven
Switched Broadcast
CN CN
Circuit-Switched Packet-Switched
CN CN
shared medium
MAC
Switched networks
Links are point-to-point
Internet)
Routing becomes harder
Switched Network
Switched Network is a series of interlinked nodes which
are called switches
In a switched network, some switching nodes are
Datagram approach
Virtual circuit switched approach
Circuit Switched Network
Three phases in circuit switching
Establish
Transfer
Disconnect
The telephone message is not broken
originally
Electronic signals pass through many switches before a
available
not necessarily the shortest route
There is no fixed path
intermediate nodes for all the packets passed during the session
between the two end nodes.
In each intermediate node, an entry is registered in a table to
indicate the route for the connection that has been set up.
Thus, packets passed through this route, can have short headers,
circuit-switched network
implemented in the physical layer
A datagram network
is a datagram network
13.
29
Gigabit Ethernet implementations
The least significant bit of the first byte defines the type of address
If the bit is 0, the address is unicast; otherwise, it is multicast.
The broadcast destination address is a special case of the multicast
address in which all bits are 1s.
The source address is always a unicast address
Example
Define the type of the following destination addresses:
a. 4A:30:10:21:10:1A b. 47:20:1B:2E:08:EE
c. FF:FF:FF:FF:FF:FF
Solution
To find the type of the address, look at the second
hexadecimal digit from the left.
If it is even, the address is unicast.
If it is odd, the address is multicast.
If all digits are F’s, the address is broadcast.
a. This is a unicast address because A in binary is 1010.
b. This is a multicast address because 7 in binary is 0111.
c. This is a broadcast address because all digits are F’s.
Example
Solution
The address is sent left-to-right, byte by byte;
for each byte, it is sent right-to-left, bit by bit;
34
Wireless LAN (WLAN)
A wireless LAN uses wireless transmission medium
A Wireless WLAN
It’s a “hub” without wires
Wireless LAN
A WLAN typically extends an existing wired LAN
The access point (AP) is attached to the edge of the wired
WLANs use the 900 MHz, 2.4 GHz and 5-GHz frequency
bands.
ISM (Industry, Scientific, Medical) license-free (unlicensed)
frequency bands
infrastructure network
access point
application application
TCP TCP
IP IP
LLC LLC LLC
802.11 MAC 802.11 MAC 802.3 MAC 802.3 MAC
802.11 PHY 802.11 PHY 802.3 PHY 802.3 PHY
Components of IEEE 802.11 architecture
The basic service set (BSS) is the basic building block of an IEEE 802.11
LAN
Each of BSS1 and BSS2 has two STAs that are members of their BSS
Think of the ovals as coverage area of a BSS within which the member STAs
may remain in communication
This area is called the Basic Service Area (BSA).
If a STA moves out of its BSA, it can no longer directly communicate with other
STAs present in the BSA
Components of IEEE 802.11 architecture
Two type of BSS: Independent and Infrastructure BSS
Every BSS has an id called the BSSID, it is the MAC address of the
access point servicing the BSS
Independent BSS (IBSS) is simply comprised of one or more Stations
which communicate directly with each other (ad-hoc network)
They contain no Access Points
They can not connect to any other basic service set
Components of IEEE 802.11 architecture
In Infrastructure BSS,
STAs communicate with each other through Access Points
STAs can communicate with other stations not in the same basic
service set through Access Points
Components of IEEE 802.11 architecture
An Extended Service Set (ESS) is a set of connected BSS
Access Points in an extended service set are connected by a distribution
system
Each ESS has an ID called the SSID
An ESS is the union of the infrastructure BSSs with the same
SSID connected by a DS
Components of IEEE 802.11 architecture
A Distribution system (DS)
connects Access Points in an extended service set